Kikuji Hirose

Real-space electronic-structure calculations with a time-saving double-grid technique

We present a set of efficient techniques in first-principles electronic-structure calculations utilizing the real-space finite-difference method. These techniques greatly reduce the overhead for performing integrals that involve norm-conserving pseudopotentials, solving Poisson equations, and treating models which have specific periodicities, while keeping a high degree of accuracy. Since real-space methods are inherently local, they have a lot of advantages in applicability and flexibility compared with the conventional plane-wave approach, and promise to be well suited for large and accurate {it ab initio} calculations. In order to demonstrate the potential power of these techniques, we present several applications for electronic structure calculations of atoms, molecules and a helical nanotube.

First-principles study of electron-conduction properties of helical gold nanowires.

Multishell helical gold nanowires (HGNs) suspended between semi-infinite electrodes are found to exhibit peculiar electron-conduction properties by first-principles calculations based on the density functional theory. Our results that the numbers of conduction channels in the HGNs and their conductances are smaller than those expected from a single-atom row nanowire verify the recent experiment. In addition, we obtained a more striking result that, in the cases of thin HGNs, distinct magnetic fields are induced by the electronic current helically flowing around the shells. This finding indicates that the HGNs can be good candidates for nanometer-scale solenoids.

Geometry and conductance of Al wires suspended between semi-infinite crystalline electrodes

We present the first-principles study of the coherent relationship between the optimized geometry and conductance of a three-aluminum-atom wire during its elongation. Our simulation employs the optimum model including semi-infinite crystalline electrodes using the overbridging boundary-matching method [Phys. Rev. B 67, 195315 (2003)] extended to incorporate nonlocal pseudopotentials. The results that the conductance of the wire is

Even-odd oscillation in conductance of a single-row sodium nanowire

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First-principles study on field evaporation for silicon atom on Si(001) surface

and that the conductance trace as a function of electrode spacing shows a convex downward curve before breaking are in agreement with experimental data.

First-principles study of Peierls instability in infinite single row Al wires

We present a first-principles calculation of the electronic conduction properties of single-row sodium nanowires suspended between semi-infinite electrodes. The conductance of the nanowire is ~1 G0 (=2e^2/h) and oscillates with a two-atom period as the number of the atoms within the nanowire varies. Moreover, we observed bunches of high electron density with a two atom-lengths in the channel density distribution. The relation between the period of the conductance oscillation and the length of bunches are examined by using simplified models and is found to be largely affected by the characteristics of the infinite wire.

Magnetic orderings in Al nanowires suspended between electrodes

The simulations of field-evaporation processes for silicon atoms on various Si(001) surfaces are implemented using the first-principles calculations based on the real-space finite-difference method. We find that the atoms which locate on atomically flat Si(001) surfaces and at step edges are easily removed by applying an external electric field, and the threshold value of the external electric field for evaporation of atoms on atomically flat Si(001) surfaces, which is predicted between 3.0 and 3.5 V/A, is in agreement with the experimental data of 3.8 V/A. In this situation, the local field around an evaporating atom does not play a crucial role. This result is instead interpreted in terms of the bond strength between an evaporating atom and surface.

First-principles study on scanning tunneling microscopy images of different hydrogen-terminated Si(110) surfaces

We present the relation between the atomic and spin-electronic structures of infinite single-row atomic wires made of Al atoms during their elongation using first-principles molecular-dynamics simulations. Our study reveals that the Peierls transition takes place in the Al wire with magnetic ordering: the wire ruptures to form a trimerized structure with antiferromagnetic ordering and changes from a conductor to an insulator just before the zigzag wire transforms into the linear one of equally-spaced atoms. The formation of the trimerized wire is discussed in terms of the behavior of the σ-symmetry bands of the Al wire.

A coherent relation between structure and conduction of infinite atomic wires

A theoretical analysis of a relation between atomic and spin-electronic structures for the ground state of single-row aluminum nanowires suspended between Al(001) electrodes is demonstrated using first-principles structural optimizations. We obtain an unusual result that a three-aluminum-atom nanowire sandwiched between the electrodes does not manifest magnetic ordering, although an isolated aluminum trimer molecule in a straight line is spin-polarized. On the other hand, a five-atom nanowire exhibits ferromagnetic ordering, where three central atoms form a spin-polarized trimer. Moreover, in the case of an eight-atom nanowire, the middle atoms in the nanowire form two spin-polarized trimers with antiferromagnetic ordering.

Atomic Structure of Si(001)-c(4?4) Formed by Heating Processes after Wet Cleaning and Its First-Principles Study

Scanning tunneling microscopy images of various hydrogen-terminated Si(110) flat surfaces and surfaces with steps, isolated monohydride row, and missing monohydride row are studied using first-principles calculations. Our results show that the calculated filled-state images and local density of states are consistent with recent experimental results, and the empty-state images appear significantly different from the filled-state ones in every model. To elucidate the origin of this difference, we examined in detail the local density of states, which affects the images, and found that the difference is largely attributed to the characteristics of the bonding and antibonding states of surface silicon atoms.